An image forming apparatus having color copying capability is provided with photosensitive drums for the colors yellow, magenta, cyan and black. A laser beam is applied to the surfaces of these photosensitive drums, and electrostatic latent images are formed on the surfaces of the photosensitive drums. The electrostatic latent images are developed with developers for the colors yellow, magenta, cyan and black, and become visible images. The visible images are transferred to a transfer belt, which is moved in making contact with the surfaces of the photosensitive drums. The visible images of each color transferred to the transfer belt are transferred to a paper sheet. The transferred paper sheet is sent to a heating roller. The heating roller heats the paper sheet to fix the visible image transferred to the surface of the paper sheet. The transfer belt is pressed to the surfaces of the photosensitive drums by transfer rollers.
In such an image forming apparatus, the transfer belt is moved continuously even after the visible image is transferred to a paper sheet, until the unnecessary developer remaining on the transfer belt is eliminated by a cleaner. If the transfer belt is held contacting the photosensitive drums in this time, the surface of the photosensitive drum is worn and the drum life is reduced. To prevent this, after the visible images of each color on the transfer belt are transferred to a paper sheet, the transfer rollers pressing the transfer belt to the photosensitive drums are moved to the opposite side of the transfer belt. By this movement, the transfer belt is separated from the photosensitive drums. After the developer remaining on the transfer belt is eliminated with a cleaner, the transfer rollers are moved to the transfer belt and the transfer belt is brought into contact with the photosensitive drums.
However, vibration is generated when the transfer rollers are moved to the transfer belt and the transfer belt is brought into contact with the photosensitive drum. This vibration is transmitted to an exposing unit which exposes the photosensitive drum to a laser beam, and a scanning unit which optically reads an image of document, affecting the image forming operation. For example, jitter or color shift occurs in an image transferred to a paper sheet.